This invention relates to calligraphic (or stroke) cathode ray tube (CRT) display systems, and more particularly to an improved system for superimposing image data received from two sources on one CRT display.
Calligraphic CRT displays (also known as stroke or vector displays) are often used in applications requiring high resolution or special purpose graphic display capabilities. The usual application employs one CRT monitor for each source of display data. There are applications in which it would be useful to mix signals from two separate sources to drive one CRT display and present superimposed images. Thus, for example, a primary equipment may produce a stroke signal for display on the CRT monitor to which additional data is to be added or superimposed from another source. If the two sources operate at different refresh rates, mixing the two sources to obtain an acceptable image quality is a problem not suitably addressed by the prior art.
A known way to mix two stroke signals into one monitor is scan conversion. With this method, each signal is displayed on a separate monitor. Each monitor display is recorded by separate raster television cameras, and the two raster outputs are mixed and displayed on a raster monitor. This method is hard to calibrate, takes up a significant amount of physical space, requires use of a raster monitor with some resulting loss of image fidelity, and is relatively expensive.
Another method to mix two stroke signals with different refresh rates is to chop back and forth between the two signals, similar to the manner in which dual traces are displayed on dual trace oscilloscopes. When the two signals have different refresh rates, this technique produces many problems, such as loss of data, flickering, visibility of retrace vectors, and the like.
It would therefore be an advance in the art to provide a technique for mixing together stroke signals from sources having different refresh rate which minimizes the loss of displayed data, flicker, and image degradation.